An ophthalmic lens blank generally has a first face with a pre-determined curvature and a second face, opposite the first face on which a desired surface contour is generated by a machining process. The overall process is generally referred to as “lens surfacing” and the overall object is to yield a finished spectacle lens so that the first and second face curvatures cooperate to yield desired optical properties. In addition to this the first and/or second faces of the lens are usually coated to provide the finished spectacle lens with an enhanced ability to resist scratching (by a “hard coating”), with a low residual reflection and a desired color (by an “antireflection coating”), and/or with certain surface properties such as hydrophobic, oleophobic and dust repelling properties (by a “top coating”). Usually also a further machining process takes place (the so-called “edging”), the aim of which is to finish-machine the edge of the spectacle lens in such a way that the spectacle lens may be inserted into a spectacle frame. In all these process steps the spectacle lens (blank) must somehow be held in the machining machine(s) and coating apparatus respectively.
In more detail, hitherto the following main process steps are usually carried out in prescription workshops: Firstly, a suitable right and/or left ophthalmic lens blank is removed from a semifinished product store. The term “semifinished” is used to mean that the spectacle lens blanks, which are usually round or oval in plan view and have not yet been edged, have already been molded, machined or in another way contoured (surfaced) on one of their two optically active faces only. The spectacle lens blanks are then prepared for the blocking operation, namely by applying a suitable protective film or a suitable protective lacquer to protect the optically active face which has already been machined or contoured, i.e. the first face or blocking face.
The so-called “blocking” of the ophthalmic lens blanks then takes place. During this, the spectacle lens blank is joined to a suitable block piece, for example a lens block according to German standard DIN 58766 or document U.S. Pat. No. 7,066,797. To this end, the block piece is firstly brought into a predefined position with respect to the protected first face of the spectacle lens blank, and then in this position the space between block piece and spectacle lens blank is filled with a molten material (normally a metal alloy or wax) or an adhesive composition that is curable, e.g., by UV or visible light, as described in the earlier U.S. Patent Application Publication 2010/0170635 by the same applicant for example. Once this material has solidified or cured, the block piece forms a holder or support for machining the second face of the spectacle lens blank. The block piece is grasped by a chuck or other suitable coupling mechanism during lens generation to provide in particular secure mounting to the profiling machine while avoiding damage to the lens.
Lens surfacing is carried out then using profiling machines which typically have a cutter of some type that is moved across the second face of the ophthalmic lens blank to give the second face its macrogeometry according to the prescription. The spectacle lens blank may be stationary or rotating during the cutting operation, depending on the particular profiling machine which is being used. Typical machining processes for surfacing spectacle lenses include single point diamond turning (as the presently preferred fine cutting process for plastic materials and described in, e.g., document U.S. Pat. No. 7,597,066 B2 by the same applicant), diamond tool fly-cutting, milling (as the presently preferred rough cutting process for plastic materials and described in, e.g., document U.S. Pat. No. 5,938,381 by the same applicant), and grinding processes, applied depending on the lens material.
Usually fine machining of the ophthalmic lenses then takes place, in which the pre-machined second face of the respective spectacle lens blank is given the desired microgeometry, as described, e.g., in documents U.S. Pat. Nos. 7,066,794 B2 and 7,278,908 B2 by the same applicant. Depending on inter alia the material of the spectacle lenses, the fine machining process is divided into a fine grinding operation and a subsequent polishing operation, or includes only a polishing operation if a polishable second face has already been produced during the pre-machining stage.
Only after the polishing operation is the ophthalmic lens blank separated from the lens block (“deblocking”) before cleaning steps are carried out. Then the coating process takes place that, depending on among other things the material of the lens blank, may include spin (or dip) coating of the deblocked spectacle lens blank so as to provide at least the second face of the lens blank with a hard coating or the like, as described, e.g., in the earlier U.S. Patent Application Publication 2008/0035053, wherein the spectacle lens blank is held in the spin coating apparatus by a lens holder that has a suction cup for instance.
The coating includes vacuum coating of the deblocked spectacle lens blank so as to provide at least the second face of the lens blank with an antireflection coating and optionally a top coating serving the above mentioned purposes. In the vacuum coating process the spectacle lens blank is clamped to a substrate carrier of a rotary carrier device that is located in a vacuum chamber in a vertically spaced relation with respect to an evaporation source for emitting a vapor stream onto the lens blank mounted on the substrate carrier, as described, e.g., in document U.S. Pat. No. 6,082,298.
After the coating step the ophthalmic lens blank usually is edged so that the spectacle lens can be inserted into a spectacle frame. To this end, the coated spectacle lens blank is blocked again, at this time however to a different, smaller block piece by an adhesive film portion for instance, as described, e.g., in document U.S. Pat. No. 6,641,466 by the same applicant. The edging process may also include the forming of bores, grooves, channels and/or bevels corresponding to the respective mounting requirements in the edge area of the spectacle lens, as described, e.g., in document U.S. Pat. No. 6,712,671 by the same applicant.
Finally, after edging and a further deblocking step the spectacle lens is cleaned again and ready for inspection and insertion or mounting to the spectacle frame.
One disadvantage of the conventional overall process as outlined above is that the spectacle lens blank needs to be deblocked after surfacing prior to coating, and then again blocked after coating prior to edging. These steps require manual operations that are time-consuming and labor-intensive.
In this connection, documents U.S. Pat. Nos. 5,210,695 A and 5,341,604 A disclose a system providing a lens blank and block assembly capable of being mounted in any of a surface generating machine, a finishing, i.e. lapping or polishing apparatus and an edging machine without requiring re-blocking of the lens in order to compensate for axis shifts, wherein the lens block is formed from a plastic material capable of being readily cut together with the lens blank in the edging machine. The proposed lens block however is not intended or suitable to be used in coating processes, in particular vacuum coating processes.
In this regard, document U.S. Patent Application Publication 2008/0132157 proposes a block piece for holding an optical lens to be machined, with a coupling part for holding in a workpiece chuck and with a holding part for fastening the lens, the latter having a convex or concave holding surface corresponding to a first side of the lens, wherein the holding surface is, according to the shape of the lens to be held, provided in the form of a negative aspherical, toric, progressive or free-form surface, and the block piece is made from a plastic material that can be machined. Although this document generally mentions that the lens can remain on the block piece during a coating process, it does not disclose or address how this could be done in a vacuum coating process in which the lens is subjected to a vacuum of, e.g., 5*10−3 mbar. It can be expected that liquids from the surfacing and cleaning processes will remain in particular in the “equalizing and pressure medium channels” provided in the proposed block piece, which liquids can excessively prolong the time required to reach, if at all, the necessary vacuum and in addition may lead to impure coating conditions resulting in an imperfect coating.
Another problem with the conventional overall process as outlined above is that, in particular if the block piece is made from a plastic material, and the block piece is supported in the surfacing machine by a collet chuck or the like which applies a radial compression force thereon, the block piece may assume a shape other than that which it naturally assumes in the absence of these forces. Such deformation may transfer to the spectacle lens blank blocked on the block piece so that the curve which is cut into the surface of the lens blank may become distorted when the block piece is removed from the chuck and the lens blank is deblocked from the block piece and resumes its natural shape. This problem becomes particularly acute in the manufacture of free-form spectacle lenses requiring very precise tolerances. Any slight distortion of the curve upon deblocking the spectacle lens from the block piece may take it out of the tolerance range of the particular prescription, thereby rendering it useless for its desired purpose and resulting in substantial waste.
Still another problem with the conventional process in prescription workshops is associated with in particular the single point diamond turning as the presently preferred fine cutting process for spectacle lens blanks made from plastic materials. This surfacing process as such is susceptible to small, but unacceptable errors at the center of rotation of the lens blank that are typically caused by errors of machine and tool calibration, as explained in great detail in document U.S. Pat. No. 7,440,814 by the same applicant. This, coupled to certain limitations of the subsequent flexible, polishing process, where it can be difficult to totally “clean up” or remove such center errors, have led to certain limitations in the amount of prism (i.e. surface tilt or shift with respect to the axis of rotation) permissible to cut and polish in such combined surfacing process. Experiments have shown that it can be relatively easy to cut and polish surfaces having 2 to 3 degrees of prism at the center with accurate centers, however greater amounts of prism at the center can pose problems.
A known method (see, e.g., document U.S. Pat. No. 6,913,356 B2) for fitting a block piece to a semifinished blank of an ophthalmic lens intended to have a particular prism generally consists of positioning the lens blank on a fixed base, in a centered and angularly defined manner, so that the finished face of the lens blank bears conjointly on a plurality of bearing points of the base, defining an orientation of the block piece relative to the lens blank, orienting the block piece in the defined manner, and fixing the block piece to the finished face while maintaining orientation, by with a castable low melting point metal alloy as the blocking material.
Once the lens blank is blocked on the block piece with the predetermined amount of prism that the spectacle lens shall have after surfacing, there is no need to generate, i.e. cut any prism during the surfacing process. However, one disadvantage of this known approach consists in the fact that, if the lens blank is blocked with a greater amount of prism, say 7 or 8 degrees of prism, the thickness of the wedge-shaped layer of blocking material between lens blank and block piece strongly varies along the prism axis. This gives rise to a different amount of shrinkage of the blocking material in the thickness direction along the prism axis when it solidifies (or cures if an adhesive composition would be used), which shrinkage in turn may bend or distort or even shift with respect to the block piece the blocked lens blank—as described in the earlier U.S. Patent Application Publication 2010/0170635 by the same applicant—so that, again, the curve which is cut into the surface of the lens blank may become distorted when the lens blank is deblocked from the block piece and resumes its natural shape. For this reason the amount of prism permissible to be blocked is also limited in the known approach.